what grade of engine oil are you using, have you adjusted the clutch to the workshop book, if you are sure these are correct, then the centre of the clutch, inside the cover has a kind of mushroom plug to stem the oil flowing into the clutch assembly, if this comes out it floods the clutch and can cause what you have.
Check this plug and make sure it is still fitted, sometimes it drops out and you find it at the bottom of the clutch cover when you remove it, carefull when you remove it not to loose it on the floor.